Plug-in hybrid cars are often associated with the fuel-sipping practicality of the Toyota Prius, Chevrolet Volt and Volvo V60. But in recent years the technology has trickled up into high-end luxury vehicles from Porsche, BMW and even a limited edition McLaren. Now Mercedes-AMG, the maker of top-performing Mercedes-Benzes with hand-crafted German engines, is exploring the use…